Radiation Isocenter Research Articles

Quality assurance for cancer patient safety: Clinical assessment for precise angles in linac during radiation therapy.

Quality assurance for stereotactic body radiation treatment requires that isocentric verification be ensured during gantry rotation at various angles. This study examined statistical parameters on Winston-Lutz tests to distinguish the deviation of angles from isocenter during gantry rotation using machine learning. The Varian TrueBeam linac was aligned with the marked lines on the Ruby phantom. Eight images were captured while the gantry was rotating at a 45° shift. The statistical features were derived from IsoCheck EPID software. The decision tree model was applied to these Winston-Lutz tests to cluster data into two groups: precise and error angles. At 90° and 270° angles, the gantry exhibits isocentric stability compared to other angles. In these angles, the most statistical features were inside the range. Most variations were observed at 0° and 180° angles. In most tests, the angles 45°, 135°, 225°, and 315° showed reasonable performance and with less variation. The comprehensive statistical analyses for gantry rotation of angles assists expert radiotherapists in determining the contribution of each feature that highly affects gantry movement at specific angles. Misalignment between radiation isocenter and imaging isocenter, tuning of the beam at each angle, or a slight change in the position of the Ruby phantom can further improve the inaccuracy that causes the most variations. Better precision can effectively increase patient safety and quality during cancer treatment.

1100: A simple method to optimise the radiation isocentre and Winston-Lutz results in linear accelerators

1100: A simple method to optimise the radiation isocentre and Winston-Lutz results in linear accelerators

Commissioning of the First MRlinac in Latin America.

To show the workflow for the commissioning of a MRlinac, and some proposed tests; off-axis targets, output factors for small fields, dose in inhomogeneities, and multileaf collimator quality assurance (MLC QA). The tests were performed based on TG-142, TG-119, ICRU 97, TRS-398, and TRS-483 recommendations as well as national regulations for radiation protection and safety. The imaging tests are in agreement with the protocols. The radiation isocenter was 0.34 mm, and for off-axis targets location was up to 0.88 mm. The dose profiles measured and calculated in treatment planning system (TPS) passed in all cases the gamma analysis of 2%/2 mm (global dose differences). The output factors of fields larger than 2 cm × 2 cm are in agreement with the model of the MRlinac in the TPS. However, for smaller fields, their differences are higher than 10%. Picket fence test for different gantry angles showed a maximum leaf deviation up to 0.2 mm. Displacements observed in treatment couch adding weight (50 kg) are lower than 1 mm. Cryostat, bridge, and couch attenuation was up to 1.2%, 10%, and 24%, respectively. The implemented tests confirm that the studied MRlinac agrees with the standards reported in the literature and that the strict tolerances established as a baseline should allow a smoother implementation of stereotactic treatments in this machine.

On the Use of the Fricke-Pluronic F-127 Gel Dosimeter for Radiation Isocenter Testing of a Medical Linear Accelerator.

This work presents a Fricke-XO-Pluronic F-127 2D radiochromic dosimeter with a flat-bed scanner for 2D reading and a dedicated data processing software package as a tool for performing coincidence testing of the radiation and mechanical isocenter of a medical accelerator. The optimal irradiation parameters were determined as follows: monitor units per beam and multi-leaf collimator gap, which are ≤750-≤2500 MU and 2-5 mm, respectively, for a cuboidal container with dimensions of 12 × 12 × 0.3 cm3. Despite the diffusion of Fe3+ ions occurring during irradiation, 2D reading can be performed at least 3 h after irradiation, without affecting the calculation performance of the coincidence test. The test was successfully performed for various irradiation settings. Overall, the Fricke-XO-Pluronic F-127 dosimeter has proven to be a potential tool for the coincidence testing of medical accelerators.

3D Polymer Gel Dosimeters with iCBCT 3D Reading and polyGeVero-CT Software Package for Quality Assurance in Radiotherapy.

Dynamically evolving radiotherapy instruments require advancements in compatible 3D dosimetry systems. This paper reports on such tools for the coincidence test of the mechanical and radiation isocenter for a medical accelerator as part of the quality assurance in routine radiotherapy practice. Three-dimensional polymer gel dosimeters were used in combination with 3D reading by iterative cone beam computed tomography and 3D data processing using the polyGeVero-CT software package. Different polymer gel dosimeters were used with the following acronyms: VIP, PAGAT, MAGIC, and NIPAM. The same scheme was used for each dosimeter: (i) irradiation sensitivity test for the iterative cone beam computed tomography reading to determine the appropriate monitor unit for irradiation, and (ii) verification of the chosen irradiation conditions by a star-shot 2D irradiation of each 3D dosimeter in the direction of performing the test. This work concludes with the optimum monitor unit per beam for each selected 3D dosimeter, delivers schemes for quick and easy determination of the radiation isocenter and performing the coincidence test.

Development of a digital star-shot analysis system for comparing radiation and imaging isocenters of proton treatment machine.

To directly compare the radiation and imaging isocenters of a proton treatment machine, we developed and evaluated a real-time radiation isocenter verification system. The system consists of a plastic scintillator (PI-200, Mitsubishi Chemical Corporation, Tokyo, Japan), an acrylic phantom, a steel ball on the detachable plate, Raspberry Pi 4 (Raspberry Pi Foundation, London, UK) with camera module, and analysis software implemented through a Python-based graphical user interface (GUI). After kV imaging alignment of the steel ball, the imaging isocenter defined as the position of the steel ball was extracted from the optical image. The proton star-shot was obtained by optical camera because the scintillator converted proton beam into visible light. Then the software computed both the minimum circle radius and the radiation isocenter position from the star-shot. And the deviation between the imaging isocenter and radiation isocenter was calculated. We compared our results with measurements obtained by Gafchromic EBT3 film (Ashland, NJ, USA). The minimum circle radii were averaged 0.29 and 0.41mm while the position deviations from the radiation isocenter to the laser marker were averaged 0.99 and 1.07mm, for our system and EBT3 film, respectively. Furthermore, the average position difference between the radiation isocenter and imaging isocenter was 0.27mm for our system. Our system reduced analysis time by 10min. Our system provided automated star-shot analysis with sufficient accuracy, and it is cost-effective alternative to conventional film-based method for radiation isocenter verification.

Investigation of Ionization Chamber Characteristics for Ultrahigh-dose-rate Scanned Carbon-ion Beams.

There are only a few studies on dosimetry with ultrahigh-dose-rate (uHDR) scanned carbon-ion beams. This study investigated the characteristics of four types of ionization chambers for the uHDR beam. We employed a newly developed large-plane parallel chamber to monitor a 208.3-MeV/u uHDR scanned carbon-ion beam with a 110-Gy/s average dose rate. The ionization chambers used were the Advanced Markus chamber (AMC), PinPoint 3D chamber (PPC), Farmer chamber (FC), and large-plane parallel chamber (StingRay). The AMC and StingRay surfaces and the PPC and FC geometric centers were aligned to the radiation isocenter using treatment room lasers. Using the voltage range stated in the instruction manuals, we obtained the saturation curves of the chambers. From these curves, we obtained the ion recombination correction factors using the two-voltage and three-voltage linear methods. The dose linearity was evaluated using five measurement points, and the chamber repeatability was verified by conducting repeated measurements for different dose values. Although all chambers, except for AMC, reached saturation when specified voltages were applied, they exhibited excellent linearity for different dose values. The ion recombination correction factors of the AMC obtained using the aforementioned linear methods were nearly 1. Additionally, all chambers exhibited excellent repeatability. Although the standard deviation of the PPC for the lowest dose was ~1.5%, those of all the other chambers were <1.0%. All ionization chambers can be used for measuring the relative dose, and absolute dose can be conveniently measured using the AMC with an uHDR carbon-ion scanned beam.

Feasibility of MPC enhanced couch as SRS/SBRT Pretreatment QA

Purpose: The purpose of this study was to investigate the feasibility of using the Winston-Lutz (WL) interchangeability with a Machine Performance Check-enhanced couch for pretreatment quality assurance (QA) of stereotactic radiotherapy (SRS) and stereotactic body radiation therapy (SBRT). The study employed the MPC with an electronic portal imaging device (EPID) to carry out geometric checks and verify the radiation isocenter. The isocenter size was assessed using the MultiMet cube and the MPC-enhanced couch module for SRS/SBRT pretreatment QA. Methods: The isocenter size of the MPC-enhanced couch module was compared to the WL measurements of the MultiMet cube. Measurements were taken at various gantry, collimator, and couch angles over a period of one month. The data from the cube were evaluated using PIPSRO and MultiMet (MMWL), including the offset targets. Various statistical tests were performed to evaluate the agreement, normality, separability, sensitivity, and specificity between the two methods. Results: The results showed isocenter sizes of 0.273 ± 0.065 mm, 0.293 ± 0.010 mm, and 0.209 ± 0.070 mm for PIPSPRO, MPC, and MMWL, respectively. The average bias was -0.0639 ± 0.1061 mm between MMWL and PIPSPRO, -0.0837 ± 0.0688 mm between MMWL and MPC, and 0.0198 ± 0.0696 mm between MPC and PIPSPRO. A Shapiro-Wilk test revealed no significant departure from normality for all tests and showed satisfactory discrimination through the area under the curve (AUC). A paired t-test analysis revealed a statistically significant difference between the mean isocenter size of the MPC and WL (MMPWL: t = 5.654, df = 29, p &lt; 0.001; PIPSPRO: t = -1.483, DF = 29, p = 0.1488), and there was no significant difference within the WL test (t = 3.008, DF = 29, p = 0.0054). Conclusion: Despite the statistical test results, there was agreement between the MPC and WL radiation isocenter size that was within the requirement of the AAPM TG 142 tolerance (±1.0 mm). The MPC proved to be accurate, reproducible, and consistent throughout the measurements, making it an appropriate and effective pretreatment QA tool for SRS/SBRT.

Commissioning Intracranial Stereotactic Radiosurgery for a Magnetic Resonance-Guided Radiation Therapy (MRgRT) System: MR-RT Localization and Dosimetric End-to-End Validation

This is the first reporting of the MRIdian A3iTM intracranial package (BrainTxTM) and benchmarks the end-to-end localization and dosimetric accuracy for commissioning an magnetic resonace (MR)-guided stereotactic radiosurgery program. We characterized the localization accuracy between MR and radiation (RT) isocenter through an end-to-end hidden target test, relative dose profile intercomparison, and absolute dose validation. BrainTx consists of a dedicated head coil, integrated mask immobilization system, and high-resolution MR sequences. Coil and baseplate attenuation was quantified. An in-house phantom (Cranial phantOm foR magNetic rEsonance Localization of a stereotactIc radiosUrgery doSimeter, CORNELIUS) was developed from a mannequin head filled with silicone gel, film, and MR BB with pinprick. A hidden target test evaluated MR-RT localization of the 1×1×1 mm3 TrueFISP MR and relative dose accuracy in film for a 1 cm diameter (International Electrotechnical Commission (IEC)-X/IEC-Y) and 1.5 cm diameter (IEC-Y/IEC-Z) spherical target. Two clinical cases (irregular-shaped target and target abutting brainstem) were mapped to the CORNELIUS phantom for feasibility assessment. A 2-dimensional (2D)-gamma compared calculated and measured dose for spherical and clinical targets with 1 mm/1% and 2 mm/2% criteria, respectively. A small-field chamber (A26MR) measured end-to-end absolute dose for a 1 cm diameter target. Coil and baseplate attenuation were 0.7% and 2.7%, respectively. The displacement of MR to RT localization as defined through the pinprick was 0.49 mm (IEC-X), 0.27 mm (IEC-Y), and 0.51 mm (IEC-Z) (root mean square 0.76 mm). The reproducibility across IEC-Y demonstrated high fidelity (<0.02 mm). Gamma pass rates were 97.1% and 95.4% for 1 cm and 1.5 cm targets, respectively. Dose profiles for an irregular-shaped target and abutting organ-at-risk-target demonstrated pass rates of 99.0% and 92.9%, respectively. The absolute end-to-end dose difference was <1%. All localization and dosimetric evaluation demonstrated submillimeter accuracy, per the TG-142, TG-101, MPPG 9.a. criteria for SRS/SRT systems, indicating acceptable delivery capabilities with a 1 mm setup margin.

A new system for in-room image guidance in particle therapy at CNAO

This paper describes the design, installation, and commissioning of an in-room imaging device developed at the Centro Nazionale di Adroterapia Oncologica (CNAO, Pavia, Italy). The system is an upgraded version of the one previously installed in 2014, and its design accounted for the experience gained in a decade of clinical practice of patient setup verification and correction through robotic-supported, off-isocenter in-room image guidance. The system's basic feature consists of image-based setup correction through 2D/3D and 3D/3D registration through a dedicated HW/SW platform. The major update with respect to the device already under clinical usage resides in the implementation of a functionality for extending the field of view of the reconstructed Cone Beam CT (CBCT) volume, along with improved overall safety and functional optimization. We report here details on the procedures implemented for system calibration under all imaging modalities and the results of the technical and preclinical commissioning of the device performed on two different phantoms. In the technical commissioning, specific attention was given to the assessment of the accuracy with which the six-degrees-of-freedom correction vector computed at the off-isocenter imaging position was propagated to the planned isocentric irradiation geometry. During the preclinical commissioning, the entire clinical-like procedure for detecting and correcting imposed, known setup deviation was tested on an anthropomorphic radioequivalent phantom. Results showed system performance within the sub-millimeter and sub-degree range according to project specifications under each imaging modality, making it ready for clinical application.

Data Driven Approach to Exactrac Setup Parameters after CBCT for Cranial Radiotherapy

The purpose of this study is to 1) Determine the congruence of shifts derived from Cone-Beam CT (CBCT) and from Exactrac dynamic KV X-rays 2) To setup action levels for Exactrac imaging to determine if patient potentially moved between CBCT and Exactrac acquisition and 3) To analyze if Exactrac alone can be reliably used for cranial setup forgoing the CBCT step MATERIALS/METHODS: Exactrac dynamic, uses in-room KV X-ray imaging that is mounted on the floor to monitor patient positioning during treatment. The congruence of imaging and radiation isocenter for the LINAC and Exactrac system was verified using the Winston-Lutz (WL) test over a period of one year. The WL pointer was set up and its coincidence with imaging and radiation isocenter was confirmed. Exactrac images were then acquired at various couch angles and the pointer deviation from Exactrac imaging isocenter is noted. Secondly, translation and rotation shifts for 175 consecutive cranial cases were examined to determine the deviation between shifts as denoted by CBCT and Exactrac. These patients were initially set up using CBCT and then once in treatment position, Exactrac KV images were acquired to check for any deviation. The deviation between the two systems was collected and a Kolomogorov-Smirnov (KS) test was performed on the data to check for the normality of the distribution. Statistical Process Control (SPC) was performed to derive action levels for Exactrac based shifts after CBCT. Based on repeated Winston-Lutz tests over a period of one year, the maximum deviation between radiation and imaging isocenters and Exactrac isocenter was 0.3mm and 0.3 deg over all couch angles. This number remained stable over the entire time period without necessitating any recalibration of Exactrac isocenter. Based on patient data for cranial cases, the mean and standard deviations for the largest shifts were (0.45 mm, 0.40 mm) for translations (range 0 - 2.03mm) and (0.44 deg and 0.32 deg) for rotations (range 0 - 2.21 degree). Using SPC, it was decided that the action level for translations and rotations could be set to 0.8mm and 0.5 deg respectively. Any deviation beyond this action level necessitates re-imaging to verify that the patient did not move in between the CBCT and Exactrac imaging acquisitions CONCLUSION: Exactrac system derived shifts show excellent agreement with CBCT. The isocentricity of the systems is maintained over a long period of time and shows no drift. Either system can be reliably used to set up cranial patients. With the added advantage of speed of acquisition, matching and shifts, the Exactrac system could replace CBCT for daily setup of patients undergoing cranial radiotherapy.

Impact of magnetic resonance imaging-related geometric distortion of dose distribution in fractionated stereotactic radiotherapy in patients with brain metastases.

The geometric distortion related to magnetic resonance (MR) imaging in adiagnostic radiology (MRDR) and radiotherapy (MRRT) setup is evaluated, and the dosimetric impact of MR distortion on fractionated stereotactic radiotherapy (FSRT) in patients with brain metastases is simulated. An anthropomorphic skull phantom was scanned using a1.5‑T MR scanner, and the magnitude of MR distortion was calculated with (MRDR-DC and MRRT-DC) and without (MRDR-nDC and MRRT-nDC) distortion-correction algorithms. Automated noncoplanar volumetric modulated arc therapy (HyperArc, HA; Varian Medical Systems, Palo Alto, CA, USA) plans were generated for 53patients with 186 brain metastases. The MR distortion at each gross tumor volume (GTV) was calculated using the distance between the center of the GTV and the MR image isocenter (MIC) and the quadratic regression curve derived from the phantom study (MRRT-DC and MRRT-nDC). Subsequently, the radiation isocenter of the HA plans was shifted according to the MR distortion at each GTV (HADC and HAnDC). The median MR distortions were approximately 0.1 mm when the distance from the MIC was < 30 mm, whereas the median distortion varied widely when the distance was > 60 mm (0.23, 0.47, 0.37, and 0.57 mm in MRDR-DC, MRDR-nDC, MRRT-DC, and MRRT-nDC, respectively). The dose to the 98% of the GTV volume (D98%) decreased as the distance from the MIC increased. In the HADC plans, the relative dose difference of D98% was less than 5% when the GTV was located within 70 mm from the MIC, whereas the underdose of GTV exceeded 5% when it was 48 mm (-26.5% at maximum) away from the MIC in the HAnDC plans. Use of adistortion-correction algorithm in the studied MR diagnoses is essential, and the dosimetric impact of MR distortion is not negligible, particularly for tumors located far away from the MIC.

Measurement of the radiation dose and radiation isocenter of the TrueBeam accelerator using 3D polymer gel dosimeters from the VIPAR family with different chemical history

This work describes the measurements of the ionizing radiation dose and the determination of the radiation isocenter using a tool consisting of three-dimensional polymer gel dosimeters and a dedicated software package. Specific polymer gel dosimeters were selected. Their inherent common feature is the possibility of high-resolution three-dimensional measurement of dose distribution in radiotherapy and the presence of the N-vinylpyrrolidone monomer in the composition alongside other substances such as the cross-linking agent N,N'-methylenebisacrylamide, oxygen scavengers, stabilizers and gel matrix. The aim of this study was to analyze the influence of the chemical history of N-vinylpyrrolidone (purity and aging) on the dose response of some dosimeters. In addition, this work seeks to answer the question of how important purity and aging of N-vinylpyrrolidone are in dosimetry applications, such as routine testing of medical accelerators.

Characterization of mechanical and radiation isocenter on an MR-guided radiotherapy (MRgRT) Linac.

In the emerging paradigm of stereotactic radiosurgery being proposed for MR-guided radiotherapy (MRgRT), assessment of mechanical geometric accuracy is critical for the implementation of stereotactic delivery. We benchmarked the mechanical accuracy of an MR Linac system that lacks an onboard detector/array. Our mechanical tests utilize a half beam block (HBB) geometry that takes advantage of the sensitivity of a partially occluded detector. Mechanical tests benchmarked the couch, MLC, and gantry geometric accuracy for an MR-Linac system. An HBB technique was used to irradiate an ionization chamber profiler (ICP) array with partial occlusion of individual detectors for characterization of MLC skew, beam divergence displacement, and RT isocenter localization. The sensitivity of the partially occluded detector's ICP-X (detector width) and ICP-Y (detector length) was characterized by displacing the detector relative to radiation isocenter by 0.2mm increments, introduced through couch motion. The accuracy of the HBB ICP technique was verified with a starshot using radiochromic film, and the reproducibility was verified on a conventional C-arm Linac and compared to Winston-Lutz. The sensitivity of the HBB technique as quantified through the dose difference normalized to open field as a function of displacement from RT isocenter was 6.4%/mm and 13.0%/mm for the ICP-X and ICP-Y orientation, respectively, due to the oblong detector orientation. Couch positional accuracy and sag was within ±0.1mm. Maximum MLC positional displacement was 0.7mm with mean MLC skew at 0.07°. The maximum beam divergence displacement was 0.03mm. The gantry angle was within 0.1°. Independent verification of the RT isocenter localization procedure produced repeatable results. This work serves for characterizing the mechanical and geometric radiation accuracy for the foundation of an MR-guided stereotactic radiosurgery program, as demonstrated with high sensitivity and independent validation.

Design and characterization of an aperture system and spot configuration for ocular treatments with a gantry-based spot scanning proton beam.

Proton therapy is a key modality used in the treatment of ocular melanoma. Traditionally ocular sites are treated using a dedicated eyeline with a passively scattered proton beam and a brass aperture. This work aims to design and characterize a beam-collimating aperture to treat ocular targets with a gantry-based spot scanning proton beam. A plastic aperture system that slides into the gantry nozzle of a spot scanning proton beam was designed and constructed. It consists of an intermediate scraper layer to attenuate stray protons and a 3D-printed patient-specific aperture positioned 5.7cm from the surface of the eye. The aperture system was modeled in TOPAS and Monte Carlo simulations were validated with film measurements. Two different spot configurations were investigated for treatment planning and characterized based on lateral penumbra, central axis (CAX) dose and relative efficiency. Alignment and leakage were investigated through experimental film measurements. Range was verified using a multi-layer ionization chamber. Reference dose measurements were made with a PinPoint 3D ion chamber. Neutron dose was evaluated through Monte Carlo simulations. Aperture alignment with radiation isocenter was determined to be within 0.31mm at a gantry angle of 0°. A single-spot configuration with a 10mm diameter aperture yielded film-measured lateral penumbras of 1mm to 1.25mm, depending on depth in the spread-out Bragg peak. TOPAS simulations found that a single spot configuration results in a flat dose distribution for a 10mm diameter aperture and provides a CAX dose of less than 106% for apertures less than 14mm in diameter. For larger targets, adding four corner spots to fill in the dose distribution is beneficial. Trade-offs between lateral penumbra, CAX dose and relative efficiency were characterized for different spot configurations and can be used for future clinical decision-making. The aperture was experimentally determined to not affect proton beam range, and no concerning leakage radiation or neutron dose was identified. Reference dose measurements with a PinPoint ion chamber were within 2.1% of Monte Carlo calculated doses. The aperture system developed in this work provides a method of treating ocular sites on a gantry-based spot scanning proton system. Additional work to develop compatible gaze tracking and gating infrastructure is ongoing.

An improved method for evaluating LINAC isocenter.

The traditional approach to medical linear accelerator (LINAC) isocenter quality assurance is to compare the radius of the LINAC isocenter, determined via analysis of Winston-Lutz (WL) dataset, to a threshold in order to approve the LINAC for clinical use. This scalar metric provides little insight into beam-to-target accuracy with gantry and couchmotion. To develop a method for verifying isocenter with increased sensitivity over traditional WL techniques by accounting for geometric errors that could normally beoverlooked. From WL images, we construct radiation beam axis and marker shift locations in a 3D coordinate system. These axes and shift positions are used to construct a new isocenter performance metric, the -matrix, which predicts direction and magnitude of beam-to-target errors across combinations of couch and gantry position. We introduce clinical isocenter as the location optimizing a cost function derived from the -matrix, which serves as an optimal target for tumor positioning. We demonstrated these techniques on a clinical LINAC with an initial randomly positioned marker which was subsequently repositioned based on the optimized clinical isocenter location. The marker shifts, radiation isocenter, and -matrix were compared before and after repositioning. We compared the new technique against typically used WL techniques using a monte carlo simulation modeling variations in LINAC geometry, marker position, and measurementnoise. The technique was successfully demonstrated on a Varian LINAC. Marker repositioning to clinical isocenter yielding an error matrix with magnitudes all below 0.81 mm. As expected, marker position had little impact on the radiation isocenter location and radius, and also had little impact on clinical isocenter location. The verification results show the accuracy of the -matrix to predict beam to tumor geometric inaccuracies. The Monte Carlo simulations demonstrate that the -matrix is more sensitive and specific for detecting potential treatment errors compared to the traditional WLtechniques. We have developed and demonstrated the usefulness of a framework for verifying isocenter based on a 3D model of the radiation beam axes and tumor movement from couch rotations, derived from 2D WL transmission images. The -matrix replaces the scalar isocenter radius as a metric of isocenter quality, providing insight into contribution of couch and radiation beams to isocenter quality, and exposing treatment errors ignored by the traditional method. The clinical isocenter provides an alternate to physical isocenter as a target for tumor positioning in cases of suboptimal LINACgeometry.

Impact of electronic portal image quality on Elekta AQUA® collimator isocenter.

The collimator radiation isocenter position determined in AQUA® v3.0 [Elekta AB, Stockholm, Sweden] software test "MLC Leaf and Jaw Position" was independently validated using an in-house MATLAB [Natick, MA: The MathWorks Inc.] script. The AQUA test determines radiation isocenter using the mean field center of nine 4 cm × 4 cm electronic portal imager (EPID) exposures at equidistant collimator angles. Impact of EPID image quality on AQUA reported isocenter for thirteen Elekta linear accelerators with Agility MLC heads were evaluated. Of the thirteen, three had visually and quantitatively identifiable artifacts. For the ten good EPID's there was a systematic 0.25 mm offset of the MATLAB calculated mean field center relative to AQUA in the X-axis and Y-axis. This corresponds to one image pixel and was found to be due to differences in software co-ordinate convention. After subtracting this offset there was no significant difference in AQUA and MATLAB calculated isocenter. For the three machines with poor image quality there was a demonstrated variation in AQUA calculated field center and therefore radiation isocenter relative to MATLAB. Restricting the region of interest (ROI) in AQUA software to only the irradiated section of the EPID brought AQUA and MATLAB result for these three machines into agreement.

Commissioning and Performance Evaluation of Varian TrueBeam Linear Accelerator

Aim: The aim of this study is to present a report on the commissioning results of the Varian TrueBeam machine with available photon energies so that it can benefit medical physicists during the entire commissioning process. Subjects and Methods: All tests were performed as per the recommendation of the Atomic Energy Regulatory Board. The beam profiles and percentage depth dose of both photon and electron beams were measured with Sun nuclear Radiation Field Analyzer and 0.125 cc ionization chamber. Output accuracy and consistency of photon and electron beams were measured using 0.6 cc cylindrical ionization chamber and 0.350 cc parallel plate ionization chamber, respectively. To measure the congruency of radiation and optical field and isocenter check with respect to gantry, collimator, and couch, self-developed films were used. Results: Isocenter shift due to gantry, collimator, and couch, rotation was found to be within ± 2-mm diameter circle. Congruency of optical and radiation fields was measured for symmetric fields 5 cm × 5 cm, 10 cm × 10 cm, 20 cm × 20 cm, and 30 cm × 30 cm for all available energy and was within ± 2 mm. Dosimetric leaf gap value for the multi-leaf collimator was measured with 0.125 cc ionization chamber and values are −0.853 mm, −1.106 mm, −1.097 mm, −0.765 mm, and −1.005 mm for energies 6 MV, 10 MV, 15 MV, 6 flattening filter free (FFF), and 10 FFF, respectively. Conclusions: All electrical, mechanical, dosimetric, and safety measurement were found to be within permissible limits. Before clinical use, the beam data are crosschecked with point dosimetry, portal dosimetry, and MapCHECK two-dimensional detector array and results are within tolerance.

Elekta Unity MR-linac commissioning: mechanical and dosimetry tests

Abstract We report the commissioning results of Elekta Unity for the dosimetric performance and mechanical quality assurance (QA), and propose additional commissioning procedures. Mechanical tests included multi-leaf collimator (MLC) positional accuracy, radiation isocenter diameter at the center and off-center position, and coincidence between the magnetic resonance (MR) image center and radiation isocenter. Comparisons between the measurements and calculations of the simple irradiated field, intensity modulated radiation therapy (IMRT) commissioning, MLC output factor ratio, validation of independent dose calculation software and end-to-end testing were performed to evaluate dosimetric performance. The average values of the MLC positional accuracy for film- and imaging device-based analysis were −0.1 and 0.3 mm, respectively. The measured radiation isocenter size was 0.41 mm, and the off-center results were within 1 mm. The coincidence was −0.21, −1.19 and 0.49 mm along the x-, y- and z-axes, respectively. The calculated percent depth doses (PDD) and profiles agreed with the measurements. The results of independent dose calculation were within the action level recommended by American Associations of Physicist in Medicine. The gamma passing rate (GPR) for IMRT commissioning was 98.6 ± 0.9%, and end-to-end testing of adapted plans showed agreement within 2% between the measurement and calculation. We reported the results of mechanical and dosimetric performances of Elekta Unity, and proposed novel commissioning procedures. Our results should provide knowledge to the physics community for enhancing the QA programs.

Overall Accuracy of Single-Isocenter Multiple Metastases SRS Treatments over Time: Comparison of Two Commercially Available Methods

<h3>Purpose/Objective(s)</h3> Single isocenter stereotactic radiosurgery (SI-SRS) is an efficient approach for dose delivery to multiple brain metastases since the treatment duration is considerably reduced, compared to the standard method of the isocenter placement at each tumor center. However, the complexity of this technique results in increased sensitivity to geometric and dosimetric uncertainties during plan delivery, thus patient-specific quality assurance procedures of high accuracy are mandatory. This study presents the investigation into the consistency of this technique, using the a 4D dose verification system and 3D printed head phantoms. <h3>Materials/Methods</h3> A treatment plan with two brain metastases was delivered four times in the period of one month on a Linac, to evaluate the reproducibility of the SI-SRS technique. The targets had a concave (2.4 cc) and a spherical (1.5 cc) shape, placed at a distance of 5 cm. Dose prescription was set to 8 Gy for both targets and a treatment plan of a single fraction using four Hyperarc beams, one full coplanar arc and three half noncoplanar arcs, arranged with a single isocenter automatically located on the center of the distance between the two targets was created. Before plan delivery, Winston-Lutz (WL) tests were performed to determine the diameter of the sphere containing the radiation isocenter for various positions of the collimator, gantry and couch. The dose distribution was recalculated on 4D geometry and measured with the 1000 SRS ion chamber array. The head phantom accommodated a film insert and the irradiation procedure consisted of positioning, imaging and plan delivery as if it was the real patient. Gamma analysis was performed for 3%/1mm acceptance criteria using a dose threshold of 1 Gy for films and 10% of maximum dose for 4D. <h3>Results</h3> The Root Mean Square (RMS) variation of the isocenter was 0.71±0.15 mm for gantry, 0.66±0.14 mm for collimator and 0.87±0.22 mm for couch, validating that the isocenter diameter was within the tolerance of 1 mm before each measurement. Gamma analysis revealed a mean passing rate (GPR) of 93.2%±0.07% for 4D measurements, compared to 91.2%±1.8% for the film ones. The reduced GPR in the case of the head phantom is expected, since it is an End-to-End QA procedure, incorporating not only the plan delivery uncertainties, but also the ones related to image registration and couch rotation. <h3>Conclusion</h3> Both QA methods showed a good agreement with each other and the calculated dose distribution having GPRs above 90% for the 3%/1mm criteria, rendering them suitable for verification of SRS plans' dosimetric and spatial accuracy. The standard deviation of the GPRs revealed that the plan delivery was consistent between the measurements, demonstrating high reproducibility and accuracy of the SI-SRS technique.